This invention relates generally to variable resistance devices and methods and, more particularly, to devices and methods which employ resistive rubber materials for providing variable resistance.
Variable resistance devices have been used in many applications including sensors, switches, and transducers. A potentiometer is a simple example of a variable resistance device which has a fixed linear resistance element extending between two end terminals and a slider which is keyed to an input terminal and makes movable contact over the resistance element. The resistance or voltage (assuming constant voltage across the two end terminals) measured across the input terminal and a first one of the two end terminals is proportional to the distance between the first end terminal and the contact point on the resistance element.
Resistive elastomers or resistive rubber materials have been used as resistance elements including variable resistance devices. The terms xe2x80x9cresistive rubberxe2x80x9d and xe2x80x9cresistive rubber materialxe2x80x9d, as used herein, refer to an elastomeric or rubber material which is interspersed with electrically conductive materials including, for example, carbon black or metallic powder. Heretofore, the use of resistive rubber in variable resistance devices has been limited to relatively simple and specific applications. For instance, some have only exploited the variable resistance characteristics of a resistive rubber caused by deformation such as stretching and compression. There is a need for variable resistance devices and methods which utilize more fully the resistive characteristics of resistive rubber materials.
The present invention relates to variable resistance devices and methods that make use of the various resistive characteristics of resistive rubber materials. The inventors have discovered characteristics of resistive rubber materials that previously have not been known or utilized.
The resistance of a resistor is directly proportional to the resistivity of the material and the length of the resistor and inversely proportional to the cross-sectional area perpendicular to the direction of current flow. The resistance is represented by the following well-known equation:
R=xcfx81l/Axe2x80x83xe2x80x83(1)
where xcfx81 is the resistivity of the resistor material, l is the length of the resistor along the direction of current flow, and A is the cross-sectional area perpendicular to the current flow. Resistivity is an inherent property of a material and is typically in units of xcexa9xc2x7cm. The voltage drop across the resistor is represented by the well-known Ohm""s law:
R=E/Ixe2x80x83xe2x80x83(2)
where E is the voltage across the resistor and I is the current through the resistor.
When resistors are joined together in a network, the effective resistance is the sum of the individual resistances if the resistors are joined in series. The effective resistance increases when the number of resistors that are joined in series increases. That is, the effective resistance increases when the total length l of the resistors increases, assuming a constant cross-sectional area A according to a specific example based on equation (1). If the resistors are joined in parallel, however, the effective resistance is the reciprocal of the sum of the reciprocals of the individual resistances. The higher the number of resistors that are joined in parallel, the lower the effective resistance is. This is also consistent with equation (1), where the effective resistance decreases when the total area A of the resistors increases in a specific example, assuming a constant length l.
Commonly available resistors typically include conductive terminals at two ends or leads that are connected between two points in a circuit to provide resistance. These resistors are simple and discrete in structure in the sense that they each have well-defined contact points at two ends with a fixed resistance therebetween. The effective resistance of a resistive network formed with resistors that have such simple, discrete structures is easily determinable by summing the resistances for resistors in series and by summing the reciprocals of the resistances for resistors that are in parallel and taking the reciprocal of the sum. Geometric factors and contact variances are absent or at least sufficiently insignificant in these simple resistors so that the effective resistance is governed by the simple equations described above. When the resistors are not simple and discrete in structure, however, the determination of the effective resistance is no longer so straightforward.
The inventors have discovered that the effective resistance is generally the combination of a straight path resistance component and a parallel path resistance component. The straight path resistance component or straight resistance component is analogous to resistors in series in that the straight resistance component between two contact locations increases with an increase in distance between the two contact locations, just as the effective resistance increases when the total length l increases and the area A is kept constant in equation (1). The increase in the amount of resistive material in the current path between the two contact locations causes the increase in resistance. The parallel path resistance component is analogous to resistors in parallel. As discussed above, the effective resistance decreases when the total area A of the combined resistors having a common length l increases. This results because there are additional current paths or xe2x80x9cparallel pathsxe2x80x9d provided by the additional resistors joined in parallel. Similarly, when the amount of parallel paths increases between two contact locations due to changes in geometry or contact variances, the parallel path resistance component decreases. As used herein, the term xe2x80x9cparallel pathsxe2x80x9d denote multiple paths available for electrical current flow between contact locations, and are not limited to paths that are geometrically parallel.
In accordance with an aspect of the present invention, a variable resistance device comprises a resistive member comprising a resistive rubber material. A first conductor is configured to be electrically coupled with the resistive member at a first contact location over a first contact area. A second conductor is configured to be electrically coupled with the resistive member at a second contact location over a second contact area. The first contact location and the second contact location are spaced from one another by a distance. A resistance between the first conductor at the first contact location and the second conductor at the second contact location is equal to the sum of a straight resistance component and a parallel path resistance component. The straight resistance component increases as the distance between the first contact location and the second contact location increases, and decreases as the distance between the first contact location and the second contact location decreases. The parallel path resistance component has preset desired characteristics based on selected first and second contact locations and selected first and second contact areas.
In certain embodiments, the first and second locations and first and second contact areas are selected to provide a parallel path resistance component which is at least substantially constant with respect to changes in the distance between the first contact location and the second contact location. As a result, the resistance between the first conductor at the first contact location and the second conductor at the second contact location increases as the distance between the first contact location and the second contact location increases, and decreases as the distance between the first contact location and the second contact location decreases.
In other embodiments, the first and second contact locations and first and second contact areas are selected such that the parallel path resistance component is substantially larger than the straight resistance component. The change in the resistance between the first conductor at the first contact location and the second conductor at the second contact location is at least substantially equal to the change in the parallel path resistance component between the first conductor and the second conductor.
In still other embodiments, the resistive member has a resistive surface for contacting the first and second conductors at the first and second contact locations, respectively. The resistive surface has an outer boundary and a thickness which is substantially smaller than a square root of a surface area of the resistive surface. The parallel path resistance component between the first conductor at the first contact location and the second conductor at the second contact location is substantially larger than the straight resistance component when both the first and second contact locations are disposed away from the outer boundary of the resistive surface. The straight resistance component between the first conductor at the first contact location and the second conductor at the second contact location is substantially larger than the parallel path resistance component when at least one of the first and second contact locations is at or near the outer boundary of the resistive surface.
In accordance with other aspects of the invention, the resistance between the first conductor at the first contact location and the second conductor at the second contact location increases when the resistive member undergoes a stretching deformation between the first contact location and the second contact location. The resistance between the first conductor at the first contact location and the second conductor at the second contact location decreases when the resistive member is subject to a pressure between the first contact location and the second contact location. The resistance between the first conductor at the first contact location and the second conductor at the second contact location increases when the resistive member undergoes a rise in temperature between the first contact location and the second contact location, and decreases when the resistive member undergoes a drop in temperature between the first contact location and the second contact location.
Another aspect of the present invention is directed to a method of providing a variable resistance from a resistive member including a resistive rubber material. The method comprises electrically coupling a first conductor with the resistive member at a first location over a first contact area and electrically coupling a second conductor with the resistive member at a second location over a second contact area. At least one of the first location, the second location, the first contact area, and the second contact area is changed to produce a change in resistance between the first conductor and the second conductor. The resistance between the first conductor and the second conductor includes a straight resistance component and a parallel path resistance component. The straight resistance component increases as the distance between the first location and the second location increases and decreases as the distance between the first location and the second location decreases. The parallel path resistance component has preset desired characteristics based on selected first and second locations and selected first and second contact areas.